It is known that the plasma etching of a layer (of oxide or polycrystalline silicon, etc.) extending over the surface of a doped or non-doped monocrystalline silicon wafer may damage the surface of the monocrystalline silicon. This is due to the fact that the monocrystalline silicon remains exposed to the plasma during an over-etching stage which is necessary to ensure complete elimination of the layer. This surface damage of the silicon may cause a considerable deterioration in the electrical characteristics of the electronic devices produced by processing of the wafer. It is therefore important to investigate this phenomenon in order to try to reduce as far as possible the thickness of the damaged layer and its adverse effects on production output and on the reliability of the devices.
One of the most important mechanisms causing damage to the silicon is "amorphization" of the first layers of the monocrystalline silicon crystal lattice due to bombardment by the ionic species of the plasma. The extent of the damage depends substantially on the intrinsic characteristics of the etching equipment and on the process parameters selected on the equipment.
A need for suitable methods of evaluating new machines or process variations in real time or, in any case, within short periods of time has been felt for some time both by manufacturers and by users of plasma etching equipment.
Various methods are known for measuring the state of disorder of the monocrystalline silicon crystal lattice, such as, for example, those based on "helium ion channeling", on "Raman scattering", and on TEM (transmission electron microscopy) measurements. These methods provide accurate information on the thickness of the damaged layer but have the disadvantage of requiring expensive measurement equipment and very long response times (several days).
A method known as the "thermal wave" (TW) technique, which permits short response times (a few minutes) is also available. This is based on a measurement of the variations (.DELTA.R) in the reflectivity (R) induced by the thermoelastic deformation generated by a laser beam striking the surface of the silicon under test perpendicularly and measured by the deflection of a second laser beam. A measurement of .DELTA.R/R provides a correlation, in arbitrary units, between the damage and the effects on the devices, but is fairly inaccurate because it depends upon the time elapsing between the etching and the measurement, on the crystal excitation frequency, and on temperature. Moreover, it cannot distinguish between the damaged silicon layer and the native oxide layer, that is, the layer of silicon dioxide which grows on the damaged silicon during exposure to air within the period of time between the completion of the plasma etching and the start of the measurement.
A method of measuring thicknesses with the use of an ellipsometer with a monochromatic light source is also known. The refractive index and the thickness of a layer which is transparent for the wavelength of the source can be obtained with this method. For an opaque layer, it is possible to obtain only the absorption coefficient (the imaginary portion of the refractive index), but not the thickness. This method can be used to measure a single layer on a larger substrate, that is, a substrate of a thickness much greater than the layer to be measured, such as a thin layer on a monocrystalline silicon wafer.